4 WATER REPTILES OF THE PAST AND PRESENT 



The many new discoveries of extinct forms so often intermedi- 

 ate, not only between the larger groups, but between many of the 

 lesser ones as well, are making the classification of the vertebrates 

 increasingly difficult. At one time it was sufficient to define a 

 reptile as a cold-blooded animal with a single occipital condyle, 

 that is, with a single articular surface between the skull and the 

 first vertebra of the neck; a mammal as a warm-blooded animal 

 with two articular surfaces; but these definitions are no longer 

 strictly correct, Coimecting links do not break down classifica- 

 tion, as one might think, but they do often spoil our fine systems 

 and compel our classi&ers to take a wider view of nature than their 

 own narrow province affords. 



We x:an never hope that most, or even the greater part, of all 

 the animals which have lived in the past will ever become known 

 to us, even imperfectly. Doubtless the species of the past geologi- 

 cal ages outnumbered many times, perhaps hundreds of times, 

 all those now living, since many of these latter are merely the 

 remnants of far more varied and extensive faunas. At times the 

 conditions for the preservation of the remains of animal Hfe have 

 been more favorable than at others, and, under such favorable 

 conditions, a fairly good glimpse is sometimes given us of the fauna 

 of some isolated epoch and locahty in the earth's history. Those 

 animals which Kved in and about the water have been preserved 

 in greater numbers and more perfectly than the strictly land 

 animals, since fossils are due to the preserving action of water, 

 with few exceptions. Of those animals which Hved upon the land 

 or in the air only the rarest of accidents carried the skeletons into 

 the lakes, seas, and oceans. And, even when they had been covered 

 by sediments at the bottoms of lakes and seas and hidden away 

 from adverse agencies, it has often happened that the great erosions 

 of later ages have carried away and destroyed the rocks in which 

 they were inclosed. The records of long intervals of time have 

 thus been lost in all parts of the world. That we are able to obtain 

 even an imperfectly continuous history is due to the fact that the 

 intervals thus lost are not everywhere contemporaneous, that the 

 missing records of one place may be filled out in part elsewhere. 

 But this substitution of records from a distance can never make 



